1. Technical Field
The present invention relates to an optical position measuring arrangement for generating n>1 phase-shifted, shift-dependent incremental signals in relation to two objects which are movable with respect to each other along a measuring direction (x), and includes a measurement grating and a scanning unit having a scanning grating arranged in the scanning plane, as well as several detector units arranged downstream of the scanning grating, from which a fringe pattern of the fringe pattern periodicity TPS results in the scanning plane because of the interaction of the bundles of beams emitted by a light source with the measurement grating and further optional gratings in the path of the scanning beams.
2. Background Information
Periodic fringe patterns are created in known optical position measuring arrangements by the interaction of a bundle of beams, emitted by a light source, with a measurement grating and possibly further gratings in the path of the scanning beam. In case of the relative movement of defined components of the position measuring arrangement, customarily the measurement grating and a scanning unit, a modulation of the intensity of the fringe pattern results, which can be evaluated for generating shift-dependent incremental signals. Such an evaluation can take place, for example, via so-called structured photo detectors, which include a periodic arrangement of photo diodes. The periodicity of the detector arrangement is matched to the periodicity of the fringe pattern as a function of the number of phase-shifted incremental signals. In the case of higher resolving optical position measuring arrangements, increasingly finer fringe patterns result, i.e. their periodicity is correspondingly reduced; then the required width of the photo diodes is also reduced in a structured photo detector used for scanning such fringe patterns. Because of restrictions as a result of their manufacture, the width of the photo diodes in such an arrangement cannot fall below a defined minimal width. No more suitable structured photo diodes are available below a fringe pattern period of approximately 40 μm.
Therefore solutions have become known where a scanning grating, which includes several different grating sections, is arranged in the scanning plane in which the periodic scanning fringe pattern results. Every grating section has a defined spatial deflection effect on the bundle of beams being propagated through this grating section. The deflection effects of different grating sections differ here. Detector elements, by which the shift-dependent incremental signals can be detected, are then assigned to the different grating sections in the respective spatial deflection direction in the detection plane. In connection with such solutions, reference is made to, for example, EP 724 713 B1. The known variation in EP 724 713 B1 is disadvantageous because of an increased susceptibility of the measurement grating to dirt, in particular linearly-shaped dirt deposits extending along the measuring direction, as well as to inhomogeneities in the measurement grating, for example scratches.